The present invention relates to a spectrum analyser. It is known to construct a spectrum analyser from a unity gain storage (delay line) loop with a phase shifter which advances the phase in the loop by means of a signal whose frequency is 1/T, where T is the delay time of the storage loop. See "Spectrum analysis with delayline filters". H. J. Bickel IRE Wessou Convention Record, pt. 8, pp. 59-67, 1959, and "High-speed Fourier Analysis with recirculating delayline-hererodyner feedback loops", J. Capon IRE Transaction on Instrumentation, pp. 32-37, June 1961.
These references describe the use of a bulk acoustic or surface acoustic wave delay line. Such acoustic delay devices suffer from a number of significant drawbacks. It is difficult to establish the desired delay line length reproducibly and changes to the delay line length are expensive and time consuming to implement. For example, in the case of a surface acoustic wave device, a new mask for the electrode structure has to be produced. These known devices also operate in a fixed frequency band and analysis of signals in other bands necessitates mixing to a fixed intermediate frequency. Acoustic delay devices are not particularly rugged and the wave velocity, and accordingly the effective delay line length, are highly temperature-dependent.